The access point (AP) in a Wireless Local Area Network (WLAN) provides a local relay function for the client devices associated with this (AP). This mode of operation is called Infrastructure Basic Service Set according to the standard specification IEEE 802.11 [ref. 1]. The Infrastructure Basic Service Set is characterized by an ID which is called ESSID. In Wi-Fi Wireless LAN computer networking, an Extended service set identifier (ESSID) is a code attached to all packets on a wireless network to identify each packet as part of that network. All wireless devices attempting to communicate with each other must share the same ESSID. Apart from identifying each packet, ESSID also serves to uniquely identify a group of wireless network devices used in a given “Service Set”.
Any WLAN client device should be able to scan the communication medium in order to detect APs transmitting at a certain channel. As long as the client device has not been connected to any AP, it can perform the scan operation as a background activity.
Background scan for 802.11 client devices non-connected to an AP should be implemented in a way that most of the device's components are kept in low power mode until certain criteria are met.
Portable apparatus supporting 802.11 Wireless Local Area (WLAN) protocol must handle efficiently their power consumption in order to increase their operation and stand by time. In some applications the WLAN functionality is enabled and disabled by the user but in many others a WLAN system always in operation and able to optimize and adjust its power consumption transparently is strongly desired. The capability of the self adjustment is essential when the WLAN apparatus is in association state, transferring data through an Access Point (AP) or just listening to the beacons, but it is also important and trickier when the device operates in a non-connected mode and performs background scan in order to discover available networks.
Background scan in non-connected mode should be the only wireless LAN network function executed by portable devices when it is in stand by mode and the user or the user's application must be notified as soon as a WLAN network has been detected. From this aspect, the power consumption due to the execution of this function affects directly the standby time of the device.
Typical examples of application that require efficient power management during the background scan of non-connected mode are those of Voice over IP (VoIP) phones and cellular phones with WLAN support. Such devices may not be connected either because no AP exist within their range or because the AP found in the same area do not fulfill certain criteria defined by the application or the user.